♦ PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦

Copyright © 2001 IET Labs, Inc.

1546 im/June, 2002

TEL: (516) 334-5959 • (800) 899-8438 • FAX: (516) 334-5988534 Main Street, Westbury, NY 11590

IET LABS, INC.Formerly manufactured by

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Standards • Decades • Strobes • Sound Level Meters • Bridges

1546 Strobotac

Digital Stroboscope

User and Service Manual

Historical NoteIET Labs continues to carry the torch lit by HaroldEdgerton in the 1930’s by his designing and makingpractical stroboscopes with extremely short flashesat very high intensity. General Radio/GenRad de-veloped an extensive line of Strobotac Stroboscopesand accessories a over a 50 year period.

The workhorse 1531 and 1538 models are still widelyused, and IET Labs supplies and supports them. Thebasic 1542 is widely used in the printing industry. TheModel 1539 Stroboslave is also built and supported.The Model 1546 is the most modern of the Stro-botacs. With its digital readout and sensitive triggerinput, it will meet nearly every challenge that a usermay impose. IET manufactures, calibrates and sup-ports all these models and others.

IET Labs is proud to maintain the tradition of qualityand excellence that is rooted in technological his-tory and now thrives in a new responsive environment.

WARNING

OBSERVE ALL SAFETY RULESWHEN WORKING WITH HIGH VOLTAGES OR LINE VOLTAGES.

Dangerous voltages may be present inside this instrument. Do not open the caseRefer servicing to qulified personnel

HIGH VOLTAGES MAY BE PRESENT AT THE TERMINALS OF THIS INSTRUMENT

WHENEVER HAZARDOUS VOLTAGES (> 45 V) ARE USED, TAKE ALL MEASURES TOAVOID ACCIDENTAL CONTACT WITH ANY LIVE COMPONENTS.

USE MAXIMUM INSULATION AND MINIMIZE THE USE OF BARECONDUCTORS WHEN USING THIS INSTRUMENT.

Use extreme caution when working with bare conductors or bus bars.

WHEN WORKING WITH HIGH VOLTAGES, POST WARNING SIGNS AND KEEP UNREQUIRED PERSONNEL SAFELY AWAY.

CAUTION

DO NOT APPLY ANY VOLTAGES OR CURRENTS TO THE TERMINALS OF THISINSTRUMENT IN EXCESS OF THE MAXIMUM LIMITS INDICATED ON

THE FRONT PANEL OR THE OPERATING GUIDE LABEL.

WARRANTY

We warrant that this product is free from defects in material and workmanship and, when properly used, willperform in accordance with applicable IET specifications. If within one year after original shipment, it is foundnot to meet this standard, it will be repaired or, at the option of IET, replaced at no charge when returned to IET.Changes in this product not approved by IET or application of voltages or currents greater than those allowed bythe specifications shall void this warranty. IET shall not be liable for any indirect, special, or consequentialdamages, even if notice has been given to the possibility of such damages.

THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUD-ING BUT NOT LIMITED TO, ANY IMPLIED WARRANTY OF MERCHANTIBILITY OR FITNESS FORANY PARTICULAR PURPOSE.

Contents

Section 1INTRODUCTION ..................................................................................................................................... 11. Purpose ..................................................................................................................................................... 1

1.2 Description ...................................................................................................................................... 11.2.1 General ................................................................................................................................... 11.2.2 Controls, Connectors and Displays ........................................................................................ 1

1.3 Accessories ..................................................................................................................................... 2Section 2

SPECIFICATIONS .................................................................................................................................... 3Condensed Operating Instructions ........................................................................................................ 3

Section 3OPERATION .............................................................................................................................................. 43.1 Power Requirements ............................................................................................................................... 43.2 Instrument TURN-ON ............................................................................................................................ 43.3 Flash Rate Adjustment ............................................................................................................................ 43.4 Speed Measurements .............................................................................................................................. 4

3.4.1 Fundamental Speed Measurements ............................................................................................. 43.4.2 Submultiple Speed Measurements ............................................................................................... 53.4.3 Measurement of Speeds Above 25,000 RPM .............................................................................. 53.4.4 Low-Speed Operation .................................................................................................................. 63.4.5 Slow-Motion Studies .................................................................................................................... 6

3.5 External Synchronization ......................................................................................................................... 63.6 Use with additional Light Sources ........................................................................................................... 7

3.6.1 Use with a Stroboslave ................................................................................................................ 73.6.2 Use with another Strobotac ......................................................................................................... 7

Section 4THEORY ..................................................................................................................................................... 84.1 Basic Stroboscope Operation .................................................................................................................. 8

4.1.1 What is a Stroboscope? ............................................................................................................... 84.1.2 Single and Multiple Images .......................................................................................................... 9

4.2 Circuit Details .......................................................................................................................................... 94.2.1 General ......................................................................................................................................... 94.2.2 The Strobotron Tube .................................................................................................................. 104.2.3 The Power-Supply Board (Figure 5-4) ...................................................................................... 104.2.4 The Digital Logic Board (Figure 5-6) ........................................................................................ 10

Section 5Service and Maintenance ........................................................................................................................125.1 Warranty ................................................................................................................................................ 125.2 Instrument Return ................................................................................................................................. 125.3 Functional Operation Checks ................................................................................................................ 125.4 Lamp Replacement ............................................................................................................................... 135.5 Etched Board Maintenance ................................................................................................................... 135.5.1 Preliminary Checks ............................................................................................................................ 135.5.2 Disassembly ....................................................................................................................................... 145.5.3 Trouble Analysis ................................................................................................................................. 145.6 Adjustment of the Internal Oscillator .................................................................................................... 17

Figures

Figure 1-1. Controls, Connectors and Display of the 1546. ................................................................................. 1Figure 4-1. 1546 Digital Strobotac Block Diagram. ............................................................................................. 9Figure 5-1. Trouble Analysis Set-Up. ................................................................................................................ 14Figure 5-2. Troubleshooting Waveforms. 1546 set to Internal mode, MED range, 3600 fpm.

117 Vac, 60 Hz line. Waveforms A through W are referenced to Vss (earth ground). .............................. 18Figure 5-2. Troubleshooting Waveforms (continued). ....................................................................................... 19Figure 5-2. Troubleshooting Waveforms (continued). ........................................................................................ 20Figure 5-3. Mechanical Parts of the 1546. ........................................................................................................ 22Figure 5-4. Etched Board Layout of the Power Supply Board (1546-4700) ..................................................... 23Figure 5-5. Schematic Diagram of the Power Supply Board (1546-4700) ........................................................ 26Figure 5-6. Etched Board Layout of the Digital Logic Board (1546-4710) ....................................................... 27

1INTRODUCTION

1546 Strobotac

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1.2.2 Controls, Connectors andDisplays

See Figure 1-1 for location of controls and connec-tors referred to in Table 1-1.

Figure 1-1. Controls, Connectors and Display ofthe 1546.

Section 1INTRODUCTION

1. Purpose

The IET 1546 Strobotac® digital stroboscope is aversatile flashing light source that is used to measurethe speed of fast-moving objects or to produce theoptical effect of stopping or slowing down high-speedmotion for purposes of observation, analysis, orhigh-speed photography.

1.2 Description

1.2.1 General

The 1546 digital stroboscope emits a high-intensity,short-duration flash of light. The instrument featuresan electronic pulse generator that controls the flashrate, a line-operated power supply, and a light-emit-ting diode (LED) readout in flashes per minute. Theinstrument has internally and externally triggeredmodes of operation. In the Internal mode, the in-strument flash is triggered by an internal oscillatorpulse which can also drive other IET stroboscopesfor additional light sources. In the External mode,the 1546 operates as a digital tachometer.

The instrument, weighing 1.25 kg (2.75 lb), is suffi-ciently light in weight to permit convenient hand-heldoperation. Thus, the light can be aimed at most mov-ing objects, including those in otherwise inaccessibleareas. The instrument is contained in a high-impact,injection-molded plastic housing. The strobe can beheld in the operator’s hands, placed on any conve-nient flat surface, or mounted on a tripod.

2 INTRODUCTION

1546 Strobotac

Table 1-1

Controls, Connectors and Display

Fig. 1-1Ref Name Use

1 EXT Switch Selects internal or external mode

2 TRIGGER IN Jack Used to input externally generated signal

3 TRIGGER OUT Jack Used to provide signal to operate a slave strobe

4 ON/OFF Switch Turns the instrument on or off

5 POWER CORD Makes connection to power line

6 LOW Range Switch Sets the instrument flash rate at 100 - 700 fpm

7 MED Range Switch Sets the instrument flash rate at 600 - 4,200 fpm

8 HIGH Range Switch Sets the instrument flash rate at 3,600 - 25,000 fpm

9 LED DISPLAY Provides digital display of flash rate

10 FLASH RATE CONTROL Adjusts the flash rate

1.3 Accessories

Table 1-2 is a list of compatible light sources that canbe used with the 1546. Refer to the IET Catalog forfurther information.

Table 1-2Additional Light Sources

1538 Strobotac® Electronic Stroboscope1539 Stroboslave® Stroboscopic Light Source1546 Stroboslave® Stroboscopic Light Source

3SPECIFICATIONS

1546 Strobotac

Flashing Rate Range: 100 to 25,000 flashes perminute (fpm) in three overlapping ranges;

Range 1 LOW 100 - 700 fpmRange 2 MED 600 - 4,200 fpmRange 3 HIGH 3,600 - 25,000 fpm

Readout Accuracy: ±0.01%, using crystal-controlledtime base.Display Resolution: ±1 fpm.Flash Duration:

Range 1 ~ 2 µS

Range 2 ~ 2 µsRange 3 ~ 1.2 µs

Tachometer Function: LED display reads fpm forboth internal and external modes. User may convertto other units of measure if required.External Trigger: Three-terminal phone jack,>+1.0 V pulse,>0.75 Vrms sine wave,or contact closure.

Trigger Output: >2.5 V in series with 1 kΩ.Power Requirements: 105 to 125 Vac, 50 to 60 Hz,20 W.Mechanical: Molded plastic case with plastic faceplate to protect lamp, diffused finish anodized-alumi-num reflector, and standard 0.250 - 20 threaded holefor tripod mounting or handle grip.Dimensions: 108 x 110 x 235 mm(4.25 x 4.3 x 9.25 in.)Weight: 1.25 kg (2.75 Ib.).Environmental:Operating Temperature: 0 - 50°C.Storage Temperature: -40 to +75°C .Humidity: 95% RH at 40°C: Vibration: 0.03 in. DA from 10-55 Hz; Bench han-dling: 4 in. or 45°; Shock; 30 g, 11 ms.Accessories Supplied: 3-conductor phone plug forexternal triggering.

Section 2SPECIFICATIONS

The instrument is ready for use immediately. Rotatethe large dial to vary the flash rate within each range.The digital readout will indicate accurately the num-ber of flashes per minute.

External Mode(In this model the stroboscope will flash synchronouslywith an external signal.)a. Push the EXT switch in.b. Plug the phone plug (included) into the jack labeledIN after the other end of the synchronizing cable hasbeen connected to an appropriate trigger signal.

Condensed Operating Instructions

GeneralPlug power cord into a standard 105-125 Vac, 50 - 60 Hz grounded receptacle.Push the ON/OFF switch in.

Internal Modea. Be sure that the switch labeled EXT is out.b. Select the desired flash rate from among the 3 over-

lapping ranges listed below.

HIGH 3600 - 25,000 fpmMED 600 - 4200 fpmLOW 100 - 700 fpm

4 OPERATION

1546 Strobotac

Section 3Operation

3.1 Power Requirements

The 1546 operates from a line frequency of 50 to 60Hz, 105 to 125 Vac, and requires 20 W of power asnoted on the panel of the instrument.

3.2 Instrument TURN-ON

WARNINGThe power plug has 3 terminals. Operatorsafety requires that the power receptacle beproperly grounded.

To turn the 1546 on:a. Connect the power cord to a power receptacle.

Ensure that the power source used corresponds tothe data on the instrument panel.

b. Push the ON/OFF switch in. The stroboscope isready for use immediately.

3.3 Flash Rate Adjustment

The 1546 can be adjusted to flash at any rate be-tween 100 to 25,000 flashes per minute (fpm). Toadjust the instrument flash rate:a. Select the appropriate range for operation fromamong the 3 overlapping ranges of the instrument.These ranges are: 100 to 700 fpm, 600 to 4,200 fpm,and 3,600 to 25,000 fpm.b. Turn the flash rate control on the top of the instru-ment until the motion of the object under observationappears stationary. The control turns continuously withno stop. When the control is turned clockwise, theflash rate increases until a point is reached when theflash rate jumps abruptly from maximum to minimumfor the range selected.

When the dial is turned counterclockwise, the flashrate decreases until it jumps from minimum to maxi-mum.

Because the control has no stops, when the maxi-mum flash rate is reached in the low and mediumranges, the flash rate can be increased by continuedclockwise rotation of the control and selection of thenext higher range. Conversely, when the mini-mum flash rate is reached in the high and mediumranges, the flash rate can be decreased by continuedcounterclockwise rotation of the control and selec-tion of the next lowest range.

3.4 Speed Measurements

3.4.1 Fundamental Speed Measurements

When measuring the rotational speed of an object,set the flash rate initially to a higher setting than theestimated speed of the object. Then, slowly reducethe flash rate until the first single image appears. Atthis point, the strobe flash rate is equal to the rota-tional speed of the object, and the speed can be readdirectly from the digital display.

When using the middle - or low-speed ranges, switchto the next higher range without moving the settingon the potentiometer to determine whether the stro-boscope is flashing at the fundamental speed of theobject. Since the ratio between ranges is exactly 6:1,6 images will appear at the next higher range, if thestrobe has been set to the fundamental speed. If only3 images appear, for example, then the strobe hasbeen set to only 1/2 the correct flash rate. On theHIGH-speed range, double the speed setting to checkfor fundamental speed operation.

5OPERATION

1546 Strobotac

A double image should occur when the frequency isdoubled. If the fundamental speed of the device isover 12,500 rpm, it will not be possible to check forthe correct speed setting by the method outlined above.In this case, refer to para 3.4.3.

With practice, an operator can measure the speed ofrotating objects quickly and accurately, especially whenthe approximate speed of the object can be estimated.It is necessary, however, to fully understand the fol-lowing basic principles when making speed measure-ments:• The operator must distinguish between single andmultiple images. Odd-shaped objects usually causelittle difficulty, but objects which are symmetrical inshape (gear, disc, fan, etc.) must be marked to pro-vide a visible reference (see section 4.1.2).• Multiple images will be observed when the flashrate is set to a multiple of the fundamental speed ofthe object.• When reducing the flash rate from a rate higherthan the fundamental speed of the object, the firstsingle image will be seen when the flash rate is equalto the fundamental speed.• When the flash rate is below the fundamental speedof the object, single and multiple images will be ob-served. The single images will occur at integral sub-multiples of the fundamental speed of the object (seesection 3.4.2).

3.4.2 Submultiple Speed Measurements

If the 1546 is set to flash at an integral submultiple ofthe fundamental speed of a rotating object, a singleimage will be observed. At flash rates between sub-multiples, multiple images will be observed. Table3-1, shows the number of images that are obtained atvarious flash rates (below the fundamental speed) ofa device rotating at 1800 rpm.

Note the exact numerical relationship between thenumerator of the submultiple fraction and the corre-sponding number of images seen. This relationshipwill always hold true regardless of the speeds involved.Table 3-1 lists a few of the more useful submultiplespeeds and corresponding images; many other mul-tiple images are possible (for example, 5 images willbe seen at 5/7, 5/8, etc.).

Submultiple flashing is necessary to observe or mea-sure the speed of objects moving at rates above 25,000rpm. Refer to para 2.4.3 for the method of determin-ing the fundamental speed when submultiple opera-tion is necessary.

Table 3-1

Submultiple Speed/Image RelationshipSubmultiples of Fundamental Number of FPM DialSpeed (1800 rpm) Images Seen* Setting

1 1 18005/6 5 15004/5 4 14403/4 3 13502/3 2 12003/5 3 10801/2 1 9002/5 2 7201/3 1 6001/4 1 4501/5 1 3601/6 1 300

* At dial settings above fundamental speed, only multiple images will be

observed.

3.4.3 Measurement of Speeds Above 25,000RPM

Speeds up to 250,000 rpm can be measured by mak-ing calculations based upon submultiple measure-ments. The procedure is as follows:

a. Starting at 25,000 fpm, decrease the strobe flashrate until a single image appears. Record the LEDreading and call it X.b. Continue decreasing the flash rate until the nextsingle image occurs. Record this reading and call it Y.c. Calculate the harmonic number, n, by:

and round off the value, n, to the nearest whole number.d. Calculate the fundamental speed, S, by:

S = nX

n =X - Y

Y

6 OPERATION

1546 Strobotac

The stroboscopic technique of slowing motion is use-ful in investigating the operation of a device underactual use conditions. Examples of such use includethe study of excessive vibration in a machine and theobservation of misaligned parts or vibrating reeds. Ona textile spinning frame, for example, the actual rela-tion between traveler and thread can be observedduring a complete revolution of the traveler.

3.5 External Synchronization

The flash of the 1546 can also be triggered by use ofan external signal. This signal can be produced elec-trically or mechanically using a contact-closure de-vice with contactors attached to a machine. The 1546will provide a display of the flash rate. Hence, the1546 can operate as a true digital tachometer as wellas a stroboscope. To operate the 1546 in the externalmode:

a. Turn the instrument on. Push in the EXTswitch.

b. Select the flash rate appropriate for the speedof the machine to be observed.

c. Connect the externally produced signal into theTRIGGER IN jack on the panel of the in-strument. The instrument will now flash andindicate the speed of the machine.

The input signal must be ground-based and have agreater-than +1 V swing. Do not apply more than100 V to the external input. The strobe will not flashor display if the signal frequency is greater than 75Hz for the LOW and MED ranges or 466 Hz for theHIGH range.

To use the 1546 with a contact closure device, 5.5Vdc is available at the ring of the 3-terminal inputphone jack.* By connecting the ring of the 3-terminalphone plug to one side of the contactor and connect-ing the other side of the contactor to the tip of thephone plug, the Strobotac will be triggered for eachcontact closure.

• Switchcraft part no. 267 (a 0.25 in., 3-circuit telephone plug)is compatible with the IN jack of the 1546. Switchcratt part no.40 or no. 250 (0.25 in., 2-terminal plug) is compatible with theTRIGGER OUT jack. Equivalents may be substituted.

Example:If X is 22,500 rpm, and Y is 16,800 rpm, then:

This number will always be very close to an integralvalue, limited only by reading accuracy; so round itoff to the nearest whole number (in this example, 3).Therefore, the fundamental speed is:

S = 3 x 22,500 = 67.500 rpm

3.4.4 Low-Speed Operation

The measurement of speeds on the LOW range ofthe 1546 may be difficult because of flicker resultingfrom lack of persistence-of-vision. These measure-ments are best made in a darkened environment, orwith the operator wearing dark glasses, in order toreduce the confusing effect of room lighting on thepattern observed.

Speeds below 100 rpm can be measured by means ofmultiple images. For example, if the flash rate of thestroboscope is twice the fundamental speed of thedevice, 2 images, 180° apart will appear. At 3 timesthe fundamental, 3 images, 120° apart, will appear.This multiple image technique can also be used forhigher speeds within the range of the 1546 whereflicker makes it difficult to tell when the correct flashrate is obtained. Refer to para 4.1.2.

3.4.5 Slow-Motion Studies

High-speed motion can be observed in “slow motion”if the rotating or reciprocating motion occurs at a con-stant rate. If the instrument flash rate is adjusted to asetting which is slightly lower than the fundamentalspeed of the object under observation, the object willappear to move slowly in the same direction as theactual motion, at a speed equal to the difference be-tween the actual speed of the object and the strobeflash rate. If the flash rate is set slightly higher thanthe speed of the object, the same slow motion willresult, but in the opposite direction.

16,80022,500 - 16,800

n = = 2.95

7OPERATION

1546 Strobotac

3.6 Use with additional Light Sources

A cable with a 2-terminal phone plug on each endcan be used to connect the 1546 trigger output (TRIG-GER OUT jack) to the trigger input of an IET 1538Strobotac, an IET 1539-A Stroboslave, or another IET1546 Strobotac. Refer to the Instruction Manual ofthe instrument to be used for additional applicationsand instructions.

3.6.1 Use with a Stroboslave

The 1539-A Stroboslave® stroboscopic light sourceis available for use with the 1546. The 1539 is aninexpensive, miniature, electronic stroboscope. It hasno internal oscillator for setting the flash rate, but mustbe triggered by an external device. It cannot be usedfor direct measurement of rotational speed. The smallstroboscope is suitable for high-speed photography

applications and motion studies other than tachometry.The 1539 is also used when a second light source isneeded, or when a difficult-to-illuminate object re-quires the use of a compact light source mounted onthe end of a flexible cord.

Since the 1539 has no internal oscillator, the triggersignal is supplied directly from the TRIGGER OUTof the 1546 to the INPUT jack of the 1539. The lampand reflector of the 1539 are connected to the unit bya 1.54 m (5 ft) flexible cable, to permit the lamp to bepositioned close to the moving object.

3.6.2 Use with another Strobotac

An IET 1538-A or another IET 1546 may be used aseither a slave or master to the 1546. Connect theTRIGGER OUT of the master unit to the TRIG-GER IN jack of the slave unit using the phone plugcable described above.

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1546 Strobotac

Theory

If the flash rate of the stroboscopeis slowed to 1799 flashes perminute, the dot will be illuminatedat a slightly different position eachtime the disc revolves, and the dotwill appear to move slowly in thedirection of rotation through 360°and arrive at its originalposition 1 minute later.

A similar movement, but in a di-rection opposite the rotation of thedot, will be observed if the flashrate of the stroboscope is in-creased to 1801 fpm. If desired,the rate of apparent movement canbe speeded up by further increasesor decreases in the strobe flashrate.

When the image is stopped, the flash rate of the strobeequals the speed of the moving object and, since theflash rate is known, the speed of the object is alsoknown. Thus the stroboscope has a dual purpose ofmeasuring speed and of apparently slowingdown or stopping rapid motion for observation. Thepractical significance of the slow-motion effect is that,since it is the true copy of the high speed motion, allirregularities (vibration, torsion, chattering, whip)present in the high speed motion can bestudied.

Section 4Theory

4.1 Basic Stroboscope Operation

4.1.1 What is a Stroboscope?

A stroboscope is a source of flashing light that can besynchronized with any fast, repetitive motion so thata rapidly moving device seems to stand still, or to moveslowly.

To illustrate this principle, consider the following ex-ample:

Assume a white disk with a singleblack dot mounted on the shaft ofan 1800-rpm motor.

When the disk is rotating at 1800rpm, it is impossible for the humaneye to distinguish a single imageand the dot will appear to be ablurred continuous circle.

When illuminated by the flashingstroboscope light, synchronized toflash once every revolution of thedisk (when the dot is at 3 o’clock,for example), the dot will be seenat this position - and only at thisposition - at a rate of 1800 timeseach minute. Thus, the dot willappear to “freeze” or stand still.

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1546 Strobotac

Theory

4.1.2 Single and Multiple Images

Single images will occur at the fundamental speed ofthe object under observation, and at predictable sub-multiples of the fundamental speed. Multiple imageswill be observed at various speeds above and belowthe fundamental speed. Refer to para 3.4.1 and 3.4.2.When the 1546 is used for observation purposes only,the ability to distinguish between single and multipleimages is usually unnecessary. When making speedmeasurements, however, the operator must be ableto make this distinction. Generally, odd shaped ob-jects (those which are not symmetrical) cause littledifficulty. Assume, for example, a fan with only oneblade: 1 blade will be seen when a single image oc-curs, 2 blades (180° apart) will be seen when a doubleimage occurs, 3 blades (120° apart) will be seen whena triple image occurs, etc.

When the object is symmetrical in shape (fans with 4blades, or a gear, for example), multiple images can-not always be distinguished from a single image. Thisdifficulty is overcome by upsetting the symmetry ofthe object by applying a reference mark with paint,chalk, tape, etc.

Gear not marked for speed mea-surement. Simple observation ispossible but the observer cannotbe certain if the image is single ormultiple.

A single image is observed withtape applied to one tooth of thegear.

A multiple (double) image is ob-served with tape applied to onetooth of the gear. The images are180° apart. (Stroboscope is flash-ing twice in one revolution of thegear.)

A multiple (triple) image is ob-served with tape applied to onetooth of the gear. The images are120° apart. (Stroboscope is flash-ing three times in one revolutionof the gear.)

4.2 Circuit Details

4.2.1 General

The 1546 Strobotac consists of a strobotron lamp, acharging circuit, a high-voltage power supply to chargethe discharge capacitors, a regulated low voltagepower supply, a flash rate oscillator, flash rate count-ing circuits, and 5 digit LED display. A block diagramof the circuitry is shown in Figure 4-1.

Figure 4-1. 1546 Digital Strobotac Block Diagram.

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1546 Strobotac

Theory

R64 sets the voltage swing for the comparator input.R64 sets the minimum oscillator frequency to approxi-mately 16 kHz with the rate potentiometer set for thelowest output frequency. Negative feedback to thecomparator is adjusted by the rate potentiometer,which controls the oscillator frequency over a 7:1range. Capacitor C35 is charged through R59 at avariable rate that is dependent upon the rate potenti-ometer setting. R46 limits the discharge current whenthe comparator output is low.

Oscillator Output Dividing Circuit: Three CMOSdividers establish the 3 flash-rate ranges and dividethe counter input signal by 256 to produce the flashrate. The 3 flash-rate ranges are established by 2 di-viders at the output of the oscillator. U3 and U4 eachdivide by 6, and are switched into the circuit to estab-lish the low and medium ranges. U7 divides the rate-oscillator output by 256, and is always in the circuit.Thus, the oscillator output of 16 to 110 kHz is dividedas shown in Table 4-1 to achieve the final flash rates.

Table 4-1

Range Flash Rate Divisor

LOW 100-700 fpm 9,216 (6 x 6 x 256)MED 600 - 4,200 fpm 1,536 (6 x 256)HIGH 3,600 - 25,000 fpm 256 (1 x 256)

Rate Counter and LED Display: Five light-emit-ting diodes (0.43 in., 7 segment) display the flash rateand are driven by a CMOS LSI Counter/DisplayDriver Integrated Circuit (U5). This IC functions asa complete rate counter and 7-segment LED driver.The input to the counter is 256 times the final flashrate. By counting this input rate for 0.234375 s, a dis-play readout 60 times the flash rate is obtained. Thus,the LED readout displays the flash rate in fpm (mul-tiplication by 0.234375 is mathematically equivalentto multiplying by 60/256).

A crystal-controlled clock IC (U6) drives the LSIcounter with control signals for gating, storing, reset-ting, and display multiplexing.

External Input Circuitry: A 3-terminal phone jackon the panel is provided to trigger the strobe from

4.2.2 The Strobotron Tube

The Strobotron tube contains an anode and a cathodein an envelope filled with xenon gas. A capacitor actsas a low-impedance source to supply high voltage tothe electrodes. The gas remains nonconducting untila high-voltage pulse is applied to trigger wires spacedbetween the electrodes. The trigger pulse ionizes thegas, allowing a high peak current to flow through itand generate an intense flash of white light.

4.2.3 The Power-Supply Board (Figure 5-4)

A voltage-quadrupling power supply, consisting of 4diodes (CR1 to CR4) and 4 capacitors (C13 to C16)provides the power needed to charge the storage ca-pacitors.

An optical isolator isolates signals from the high-volt-age circuits on the power board (which are refer-enced to one side of the power line) from signals onthe logic board, which are earth-grounded.

The trigger is generated by Q3, a silicon-controlledrectifier (SCR). The SCR rapidly discharges C18through the trigger transformer, T1. The secondaryof T1 produces the trigger pulse to fire the Strobotronlamp. The SCR is driven by the output of the opticalisolator.

4.2.4 The Digital Logic Board (Figure 5-6)

Low-Voltage Power Supply: A low-voltage power-supply circuit produces 5.5 Vdc to power all compo-nents on the Logic Board. Transformer T2 providesa 10 Vac signal to the Logic Board. This signal isrectified by a full-wave bridge and filtered by capaci-tors and a 5-V regulator. Resistors R50 and R51 in-crease the final output voltage to 5.5 V.

Rate Oscillator: The rate oscillator establishes theflash rate of the stroboscope in the Internal mode. Avoltage comparator oscillator controlled by R66 (theflash rate potentiometer) generates a pulsed outputof 16 to 110 kHz. Positive feedback through R66 and

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1546 Strobotac

Theory

ground-based signals and contact closures. The tipand rear section of the phone jack are directly coupledto the input buffer. Zener diode CR14, in series withR39, protects the input buffer from excessively highvoltages.

The ring of the 3-terminal phone jack is connected to5.5 Vdc to provide a voltage source for connection toone side of a contact closure. The strobe will flashfor each contact closure when the contact is con-nected between the tip of the plug and the ring of thephone jack.

A 1-shot multivibrator protects the lamp from exces-sively high flash rates. The 1-shot uses the leadingedge of the buffer output signal to discharge the ca-pacitor, C9. The positive buffer output drives the sec-ond voltage comparator negative, causing C9 to dis-charge. C9 recharges through R32 or R32 in parallelwith R31. R31 is switched into the circuit to changethe 1-shot delay time between the low and mediumranges and the high range.

When the voltage across C9 exceeds 52% of the sup-ply voltage, the third voltage comparator is drivennegative. This third comparator must be in the “low”output state for the strobe to flash. Thus, if the exter-nal input rate exceeds the rate at which C9 can besufficiently charged, there will be no flash output.

Display of the external mode flash rate is accom-plished using the display circuitry of the internal mode,plus a voltage-controlled oscillator (VCO) and phase-locked loop (U2). The VCO, operating at 1 to 120kHz, is phase-locked to the external input signal. Alow pass filter, made up of R44, R45 and C30, con-trols the loop dynamics and permits lock-up down to1.25 Hz (75 fpm). R43 and C11 set an upper limit forVCO operation. The output frequency of the VCO isequal to the external input rate times the range divi-sor. Prior to phase-lock acquisition, the flash-rate dis-play is blanked. The blanking feature uses a lock indi-cator output signal from the phase comparator. Thissignal, when filtered by the network consisting of R25,R26, C31 and CR13, inhibits the counter/display driverIC.

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Section 5Service and Maintenance

WARNINGThese servicing instructions are for use by qualified personnel only.Dangerous voltages are present inside the case of this instrument.

For safety, disconnect power plug and wait 3 minutes before opening.

5.1 Warranty

The warranty attests the quality of materials andworkmanship in our products. When difficulties dooccur, our service engineers will assist in any waypossible. If the difficulty cannot be eliminated, pleasewrite or phone our Service Department, givingfull information of the trouble and of steps taken toremedy it. Be sure to mention the type and serial num-ber of the instrument.

5.2 Instrument Return

Before returning an instrument to IET for serviceplease call our Service Department at 800-899-8438for Return Material Authorization (RMA). Include aPurchase Order Number to insure expedient process-ing. Units under warranty will be repaired at nocharge. For any questions on repair costs or ship-ment instructions, please contact our Service De-partment at the above number. To safeguard an in-strument during shipment, please use packaging thatis adequate to protect it from damage, (i.e., equiva-lent to the original packaging) and mark the box “Deli-cate Electronic Instrument”.

Return material should be sent freight prepaid to:

IET Labs, Inc.10 Dedham StreetNewton, MA 02461

Attention: Service Department

5.3 Functional Operation Checks

One method of verifying proper operation of the in-ternal mode of the 1546 is to compare the instrumentflash rate with the rotation of a motor that is synchro-nized to the power-line frequency. To make the veri-fication, set the flash rate of the 1546 initially higherthan the speed of the motor (generally 1800 rpm) andlower the flash rate until motion is stopped. The num-ber displayed on the LED readout should be within±0.1% of the nominal synchronous motor speed. Foran 1800 rpm motor, this would be ±1.8 rpm/fpm. Ifthe actual value is outside of the specified tolerance,the instrument most likely has a defective crystal os-cillator or counting circuit.

At times, the power line frequency may drift, so thatthe stable 1546 counter may correctly show that theflash rate is out-of-spec. When in doubt, measurethe power-line frequency with a frequency counterto determine where the problem lies, or observe themotor over an extended period of time to verify thatthe powerline frequency is drifting.

To verify the rate when using the EXT mode, input apulse of a known repetition rate from a signal gen-erator. Compare this input with the display reading.The value from the display should be within ±0.01%the value of the input. If the value isnot within this specification, or if no display is present,refer to para. 5.5.3.

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Reference Designator Abbreviations

C = Capacitor R = ResistorCR = Diode S = SwitchDS = Lamp T = TransformerF = Fuse U = Integrated CircuitJ = Jack VR = Diode, ZenerL = Inductor X = Socket for Plug-InP = Plug V = CrystalQ = Transistor Z = Network

Mechanical Parts List

Figure 5-3Ref. Description Part No. Quan.

1 Dial asm. 1546-1020 12 Friction washer 1546-7400 1

(under dial)3 Housing, uppercase asm. 1546-1010 14 Front-cover screws 7044-1102 4

.112-40 2A, .250 in5 Front-cover 0 rings 5855-0062 8

1/16 in diam. nom.6 Front Cover 1546-7030 17 Reflector asm. 1546-1030 18 Housing, lowercase asm. 1546-1000 19 Housing screws 7044-1412 4

.190-32 2A, 2.5 in (hidden)10 Lock washers for 8040-2400 4

housing screws (hidden)11 Power cable 1546-0230 112 Strain-relief clamp 4350-0800 1

for power cable (hidden)13 Push buttons for switch 5511-0402 5

(hidden)

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Figure 5-3. Mechanical Parts of the 1546.

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Electrical Parts ListPower Supply PC Board P/N 1546-4700

REFDES DESCRIPTION PART NO.

C13, 14 CAP ALUM 35 µF 375 V 4450-6166C15 CAP ALUM 7 µF 400 V 4450-6168C16 CAP ALUM 80 µF 200 V 4450-6167C17 CAP CER MONO 0.1µF 20% 50 V 4400-2050C18 CAP MYLAR .1 µF 10% 200 V 4860-8253C25 CAP POLYPR. 0.11 µF 10% 750 V 4860-1220C26 CAP, POLYPR., 0.66 µF, 10%, 750 V 4860-1230C32 CAP TANT 22 µF 20% 35 V 4450-5612

CR1-5, 16 RECT 1N4006 800PIV .5A SI 6081-1004CR14 ZENER 1N5250B 20V 5% ,400 mW 6083-1018

ALTERNATE PART 1N4747A

F1 FUSE, SLOW BLOW, ¼ A, 250 V, PIGTAIL, 2 AG 5330-4005F2 FUSE, FAST BLOW, ½ A , SUBMIN. 5330-4360

J1 CONNECTOR MULT PIN .045DIA 4230-4612

Q1 TRANS, 2N3414 or 2N3416 8210-1290Q3 SCR, C203D/2N5064 8210-1215

R1, R25 RES MOX 1 M 5% 2 W 500077-1R2, 9 RES COMP 51 K OHM 5% 1/2 W 6100-3515R3 RES COMP 33 K 5% 2 W 6120-3335R4 RES MF 10 K 5% 1 W 6110-3105R5 RES COMP 100 OHM 5% 1/4 W 6099-1105R8 RES COMP 10 OHM 5% 1/4 W 6099-0105R10 RES COMP 10 K 5% l/4 W 6099-3105R11 RES COMP 100 K 5% 1/4 W 6099-4105R54 RES COMP 200 OHM 5% 1/2 W 6100-1205R55 RES COMP 1 M 5% 2 W 6120-5105R56 RES PWR WW 10 OHMS 10% 2 W 6620-2201

S5 SWITCH PUSHBUTTON DPDT 7870-1573

T1 TRANSFORMER, TRIGGER 1542-0410T2 TRANSFORMER, POWER 7997-0400

U9 IC, PHOTO-ISOLATOR, MCT2 5434-0108

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Flash Tube Socket Asm P/N 1546-2100

REFDES DESCRIPTION PART NO.

C19-24, 40 CAP CER DISC 22 pF 20% 4000 V 4428-3116

CR1-4 RECT 1N4006 800 PIV .5A SI 6081-1004

Reflector Asm Complete P/N 1546-2200

REFDES DESCRIPTION PART NO.

R6, 7 RES WW 4.7K OHM 5% 10 W 6640-2475

XV1 FLASH TUBE SOCKET ASM 1546-2100

Note: Preferred replacements for carbon composition resistors are either carbon film or metal film resistors.

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Electrical Parts ListLogic PC Board P/N 1546-4710

REFDES DESCRIPTION PART NO.

C1-6,12,28,36,37 CAP CER DISC .01 µF 80/20 % 100 V 4401-3100C7,8 CAP CER DISC 22 pF 5% 510 V 4404-0225C9 CAP MYLAR .1 µF 2% 100 V 4860-8351C10,34 CAP CER MONO .047 µF 20% 50 VGP 4400-2040C11 CAP MICA 274 pF 1% 500 V 4710-0448C27 CAP ALUM 680 µF 15V 4450-6015C29 CAP CER MONO 0.1 µF 20% 50 VGP 4400-2050C30, 31 CAP CER MONO 1 µF 20% 50 VGP 4400-2070C35 CAP MICA 1000 pF 1% 500 V 4710-0100

CR7,13,15 DIODE IN4151 75 PIV IR.1UA SI 6082-1001CR9-12 RECT IN4004 400 PIV .75A SI 6081-1002CR14 ZENER 1N750A 4.7V 5 % .4W 6083-1028CR17 ZENER IN753A 6.2V 5 % .4W 6083-1006

Q1,2 TRANSISTOR MPS-A14 8210-1246

R14 RES COMP 1.0 K 5 % 1/4 W 6099-2105R15,16 RES COMP 47 K 5 % 1/4 W 6099-3475R17-23 RES COMP 100 OHM 5 % 1/4 W 6099-1105R24,35,38-40 RES COMP 10 K 5 % 1/4 W 6099-3105R25 RES COMP 15 K 5 % 1/4 W 6099-3155R26,34,44 RES COMP 470 K 5 % 1/4 W 6099-4475R27,30,43 RES FLM 10.0 K 1 % 1/8 W 6250-2100R28 RES FLM 100 K 1% 1/8 W 6250-3100R29 RES COMP 1.2 K 5 % 1/4 W 6099-2125R31 RES FLM 31.6 K 1 % 1/8 W 6250-2316R32 RES FLM 178 K 1 % 1/8 W 6250-3178R33,42 RES COMP 100 K 5 % 1/4 W 6099-4105R36 RES COMP 2.4 K OHM 5 % 1/4 W 6099-3245R37 RES COMP 200 K OHM 5 % 1/4 W 6099-4205R41 RES COMP 3.0 K OHM 5 % 1/4 W 6099-2305R45 RES COMP 2.7 K 5 % 1/4 W 6099-5275R46 RES FLM 2.05 K 1 % 1/8 W 6250-1205R47 RES FLM 1 K 1 % 1/8 W 6250-1100R48 RES FLM 2.37 K 1 % 1/8 W 6250-1237R49 RES FLM 1 K 1 % 1/8 W 6250-1100R50 RES COMP 30 OHM 5 % 1/4 W 6099-0305

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Electrical Parts List (continuation)Logic PC Board P/N 1546-4710

REFDES DESCRIPTION PART NO.

R51 RES COMP 360 OHM 5%1/4 W 6099-1365R52,57,58, RES COMP 10 K 5% 1/4 W 6099-310560, 61, 65R53,63 RES COMP 1.0 K 5% 1/4 W 6099-2105R56 RES COMP 100 K 5% 1/4 W 6099-4105R59 RES FLM 30.1 K 1% 1/8 W 6250-2301R62 RES COMP 75 OHM 5% 1/4 W 6099-0755R64 POT CERM TRM 1 K 20% 1T 6049-0106R66 POT COMP KNOB 5 K OHM 10% 1 W 6045-0470R67 POT CERM TRM 200 OHM 20% 1T 6049-0104

SW1-4 SWITCH PUSHBUTTON MULT 4 SECT 7880-1546

U1 IC LINEAR LM339N 5432-1065U2 ICD, CD4046BE (STATIC PROTECT REQ) 5431-7064U3,4 ICD, CD4018BE (STATIC PROTECT REQ) 5431-7063

U5 ICD, M7208 (STATIC PROTECT REQ) 5431-7216U6 ICD, M7207A (STATIC PROTECT REQ) 5431-7215U7 ICD, MC14040BCP (STATIC PROTECT REQ) 5431-7018U8 IC LINEAR LM342P-5 5432-1058U10-14 READOUT, LED, 7-SEGMENT, .43 INCH HT 5437-1310

Y1 CRYSTAL 2.236962 MHz 5075-1101

CAUTIONIC’S U2-7 are static-sensitive. Use standard precautions when servicing.